1. Field of the Invention
The present invention relates to a drive circuit for a liquid crystal display apparatus (LCD), and more particularly to a drive circuit having a simple circuit configuration for performing brightness adjustment and sub-brightness adjustment.
2. Description of the Prior Art
An LCD has a configuration in which liquid crystals are filled between electrode substrates, where transparent electrodes are formed on transparent substrates. Liquid crystals are electro-optically anisotropic so that by applying a desired voltage across the electrodes to generate an electric field on the liquid crystals, the liquid crystals exhibit optical characteristics in accordance with the electric field strength. By utilizing this property and applying a different voltage on every pixel, a display picture is generated as a collection of pixels with each pixel having a desired brightness. Thus, with pictures generated from voltage control, the LCD has advantages in terms of compact size, thin form, low power consumption, and so forth, and has increased practical applications in the fields of computer equipment and audio-visual equipment.
FIG. 1 shows a configuration of this sort of LCD module comprising a signal processor circuit 1 for performing brightness control, contrast control, gamma correction, and so forth, for input RGB digital data VD, a D/A converter 2 for converting the digital data VD that is output from the signal processor circuit 1 into analog signals, an RGB driver 3 for switching an inverting/non-inverting amplification of the RGB analog image signals that are output from the D/A converter 2, and an LCD 4.
FIG. 2 shows a brightness control circuit within the signal processor circuit 1. The RGB digital data VD and brightness data BD are supplied to an adder circuit 15 and added. Here, the brightness data BD is, for example, 8-bit data generated in accordance with an external adjustment knob, a key input, or the like. The digital data VD is, for example, 8 bits, and the brightness data BD is added, for example, to a position from the third bit of the digital data VD so that a corrected 10-bit digital data is output. The same brightness control circuit is provided for each of the R, G, and B colors.
Since the tint changes if there are variations among the individual RGB lines, the RGB driver 3 is provided with a sub-brightness adjustment terminal (not shown) formed from a variable resistance to allow adjustments for every IC.
Thus, when performing detailed brightness control, the correction data to be output from the brightness control circuit has a higher number of bits than the input data. Therefore, the size of the circuit of the D/A converter 2 increases and its cost increases. On the other hand, if the number of bits is decreased in order to avoid an increase in cost, the picture quality deteriorates.
Although the sub-brightness adjustment terminal is formed from an external variable resistance or the like, miniaturization of the package was hindered and the manufacturing cost increased.
Furthermore, although it is also possible to use a D/A converter with a built-in variable resistance and to have it controlled by a microcomputer, a D/A converter becomes necessary for each of the R, G, and B colors so that an increase in circuit size still cannot be avoided.
It is therefore an object of the present invention to solve the aforementioned problems and to provide a drive circuit capable of brightness control without any increase in circuit size.
In order to achieve this object, the present invention has the following features.
First, in one aspect of the present invention, a drive circuit for display apparatus, which comprises a plurality of display elements, comprises: a reference voltage control circuit for controlling a reference voltage level on the basis of brightness control signal in a blanking period of an input image signal; a clamping circuit for clamping the reference voltage level of a signal output from the reference voltage control circuit; and an amplifier circuit for amplifying a signal output from the clamping circuit.
In this manner, by controlling the reference voltage level in accordance with the brightness level and by clamping the reference voltage level, the DC voltage component of the image signal can be controlled, thereby making it possible to perform brightness control without an increase in circuit size.
In another aspect of the present invention, the drive circuit for display apparatus, which comprises a plurality of display elements, comprises: the reference voltage control circuit for outputting, instead of the input image signal during the blanking period of the input image signal, the reference voltage data generated on the basis of the brightness control signal, and outputting, during an effective display period of the input image signal, the input image signal; the clamping circuit for converting the reference voltage level in the blanking period of the signal output from the reference voltage control circuit into a predetermined brightness reference level; and the amplifier circuit for amplifying the signal output from the clamping circuit.
In this aspect, the brightness reference level can be adjusted by switching the signal to be output to the clamping circuit with the blanking period and the effective display period of the input image signal between the input image signal and the reference voltage data.
In the above-mentioned aspect, the reference voltage control circuit may have a configuration comprising: a reference voltage data generator circuit for generating the reference voltage data corresponding to the reference voltage level on the basis of the brightness control signal; and a switching circuit for switching, in accordance with the timing signal, the input image signal and the reference voltage data to be output.
This sort of simple circuit configuration enables the output to be switched so that the control of the DC voltage component of the input image signal can be easily executed without an increase in circuit size.
Also for the case of digital signal processing, if the digital reference voltage data generated on the basis of the digital brightness control signal is output instead of the digital input image signal, the brightness reference level can be adjusted without an increase in the number of data bits.
In another aspect of the present invention, the drive circuit for display apparatus, which comprises a plurality of display elements, comprises: the reference voltage control circuit for controlling the reference voltage level on the basis of the brightness control signal and sub-brightness control signal in the blanking period of the input image signal; the clamping circuit for clamping the reference voltage level of the signal output from the reference voltage control circuit; and the amplifier circuit for amplifying the signal output from the clamping circuit.
In this manner, controlling the reference voltage level in accordance with the brightness level and sub-brightness level and clamping the reference voltage, enables the DC voltage component of the image signal to be controlled, thereby making it possible to perform brightness control without an increase in circuit size.
In another aspect of the present invention, the drive circuit for display apparatus, which comprises a plurality of display elements, comprises: the reference voltage control circuit for outputting, instead of the input image signal during the blanking period of the input image signal, the reference voltage data generated on the basis of the brightness control signal and sub-brightness control signal, and outputting, during the effective display period of the input image signal, the input image signal; the clamping circuit for converting the voltage level in the blanking period of the signal output from the reference voltage control circuit into a predetermined brightness reference level; and the amplifier circuit for amplifying the signal output from the clamping circuit.
In this aspect, the signal to be output to the clamping circuit is switched with the blanking period and effective display period of the input image signal between the input image signal and the reference voltage data generated on the basis of the brightness and sub-brightness control signals. The brightness reference level of the image signal can be adjusted by the reference voltage data that is output as a result of this switching operation.
In the above-mentioned aspect, the reference voltage control circuit may have a configuration comprising: the reference voltage data generator circuit for generating the reference voltage data corresponding to the reference voltage level on the basis of the brightness control signal; and the switching circuit for switching, in accordance with the timing signal, the input image signal and the reference voltage data to be output.
Since the switching of the output can be realized in this manner through a simple configuration of the reference voltage control circuit, the control of the DC voltage component of the image signal can be executed without an increase in circuit size.
Also for the case of digital signal processing, if the digital reference voltage data generated on the basis of the digital brightness control signal is output instead of the digital input image signal, the brightness reference level can be adjusted without an increase in the number of data bits.
With regard to the above-mentioned drive circuit in another aspect of the present invention, the above-mentioned sub-brightness control signal is individually set for each of the red image signal, green image signal, and blue image signal for color display.
In the display apparatus, such as a liquid crystal display apparatus, the transmittance differs for each of the R light, G light, and B light. In this case also, the brightness of the image signal for every color can be adjusted with the sub-brightness control signal. Furthermore, the brightness adjustment for every color can be realized in a circuit configuration where the reference voltage data generated on the basis of the brightness and sub-brightness control signal is output instead of the input image signal during the blanking period. Therefore, a sub-brightness adjustment terminal, which leads to an increase in circuit area, is unnecessary, thereby preventing an obstacle to miniaturization of the package and preventing an increase in manufacturing cost.